In-building location determination is inherently difficult and generally not possible with the Global Positioning System (GPS) because the signals from the GPS orbital satellite signals are too weak to be received through most building construction materials. Shorter-range wireless technology can be used to determine inter-station range using either received signal strength indication (RSSI) or time-of-flight measurements. The former is typically more error prone due to the vagaries of signal reflection and ferrous materials that are capable of propagating a signal, thereby distorting what would otherwise be a relatively straightforward calculation based on an inverse square law. Time-of-flight based on the propagation time of electromagnetic (EM) waves in space may be a more accurate approach than an RSSI approach, however, timing radio signals to a few nanoseconds may be technically challenging.
Indoor location tracking utilizing the RSSI approach typically involves two or more receivers for RSSI location awareness. Using two or more receivers may be, however, an issue because the device that is a master device generally can only have eight slaves in a piconet system, so as a result finding a position for a mobile device means that each stationary device is a slave to a mobile master. If portability is an issue then duty cycle can adversely expend the battery life of the mobile devices. Also other devices may not be able to communicate with the master to save data. An alternative architecture would be to have the mobile device function as a slave in the network. The issue is, however, that the other slaves can not speak to the mobile slave.
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